The present invention relates to navigation and guidance of vehicles and/or projectiles. Specifically, the present invention relates to the navigation and guidance of rotating vehicles under conditions of RF interference or jamming. More particularly, the present invention relates to interference-aided navigation with cyclic jammer cancellation.
In many applications, such as artillery shell or missile guidance, the vehicle (e.g., munitions, ordnance) to be guided is spinning rapidly. Guidance systems integral to such spinning vehicles require significant real-time processing capacity because all of the sensor inputs and guidance actuator controls must be corrected to account for the effects of the spinning body. Neutralizing these unwanted rotational artifacts can only be accomplished if the orientation of the spinning body can also be determined in real-time. Practically speaking, it is difficult to determine the rotational orientation of a spinning body. Gyros have traditionally been used to sense orientation. Use of gyros in these kinds of applications is problematic because the scale-factor errors exhibited by spin-axis gyroscopes result in significant cumulative attitude estimate error.
Other techniques for determining the attitude of a spinning vehicle have included the use of spinning accelerometers or strain sensors mounted on a spinning wheel. These prior art apparatus provide for the measurement of the vehicle""s rotation-rate. Measurements, utilizing spinning accelerometers are referred to as Coriolis rate measurements because of sinusoidal Coriolis acceleration artifacts they include. To remove these artifacts, the sensor signals must be demodulated. This demodulation normally involves a secondary sensor that indicates the rotational position of the spinning wheel relative to the body of the spinning vehicle. Such secondary rotational orientation sensors might include sun trackers or magnetic field sensors. These types of sensors may not be appropriate for every environment and they may not work well in certain geographic regions when the vehicle is aligned with the earth""s magnetic field.
Certain difficulties are encountered in creating an effective navigation system for a rapidly spinning vehicle such as an artillery shell. Some researchers have proposed a system utilizing a magnetic field sensor for tracking the rotation angle of the vehicle and a system for computationally de-spinning the vehicle to greatly simplify calculation, and improve accuracy, of the navigation solution. Others have proposed utilizing a magnetic field sensor to de-spin the body-axis frame measurements and, in addition, used accelerometers to measure the Coriolis accelerations due to rotation. This proposal thereby eliminated the need for gyros and their associated rate range and scale factor limitations.
Use of a magnetic sensor, for roll determination in a spinning vehicle, can be effective under the correct circumstances. However, this approach requires the addition of a magnetic sensor and performance can be dependent on the magnetic properties of the vehicle and its electrical systems, as well as its position on the earth and the magnetic environment. For example, near the equator, a vehicle traveling approximately due north or south will have difficulty determining its rotation angle using a magnetic sensor.
In general, xe2x80x9cinterference signalsxe2x80x9d as referred to in the present application are external signals intended to jam or interrupt directional equipment on a vehicle. The interference signals received by a directional antenna on a rotating vehicle are typically of unknown or random amplitude and phase, but the power of the received signals are modulated as a function of the vehicle""s rotation. It is possible to correlate the power modulation of the interference signal with the expected modulation over the possible range of rotation rates to determine the frequency and phase of the rotation relative to the interference source. The exact absolute phase of rotation is not initially known because the direction of the interference source is not generally known. However, once the rotation rate is determined, the system may systematically seek the desired navigation signal in directions relative to the interference source.
In the case of GPS (global positioning system), the navigation signals tend to be available from a number of different angles. Therefore, once the relative direction of the interference source or jammer is determined, the vehicle may search for valid GPS signals in directions generally opposite the jammer. If the vehicle contains multiple antenna elements, the output of these elements may be combined to significantly reduce the total gain in the direction of the interference source. This configuration is often referred to as a xe2x80x9cnull-steered antennaxe2x80x9d because low-gain area or null of the composite antenna reception pattern is adjusted or steered to fall in the direction of the interference source. Techniques of antenna-array null-steering are well known. The basic concept is that the outputs of the individual antenna elements are phase and amplitude adjusted and combined such that the composite antenna pattern has very low gain in the direction of interference and higher gain in the direction of the desired signal.
FIG. 1 shows a block diagram for a conventional GPS receiver. The antenna signal is fed into a RF processing block that amplifies, filters, and mixes the signal down to an intermediate frequency signal SIF(t) that can be sampled by an A/D converter. The digitized output intermediate frequency signal SIF[n] is fed to the GPS signal processing where the navigation signal is extracted and tracked.
FIG. 2 shows a block diagram of a conventional beam forming system for non-spinning vehicles. The system consists of multiple antennas where each is connected to its own RF signal processing and A/D converter. The A/D converters generally have a high sample rate and dynamic range to accommodate both the desired and the much larger interference signal. Each of the AID outputs may be adjusted in amplitude and phase by a modulation function (M on the figure). A null-steering controller adjusts the commands to the individual modulators so that the summed output has minimum gain in the direction of the jamming source, and acceptably high gain in other directions. With the interference signal significantly reduced, the dynamic range, sample rate, and IF frequency may be reduced to accommodate the limitations of the GPS signal processing.
U.S. Pat. No.6,520,448, assigned to the assigned of the present application, describes a spinning-vehicle navigation system (ASVN) utilizing GPS or similar navigation signals to determine the rotation angle of the vehicle. This approach offers significant improvement in performance, and robustness under interference, once the navigation signals are acquired.
The applicant of the present application has also described a system to utilize signals from an interference source to determine the relative roll angle of the vehicle in co-pending patent application Ser. No. 10/123,928, (Atty. Dkt. No. 02CR169/KE) entitled xe2x80x9cInterference-Aided Navigation for Rotating Vehicles.xe2x80x9d The applicant also describes how this rotation information may be utilized to drive a temporal beam former to enhance signal to noise performance of the system in issued U.S. Pat. No. 6,587,078, assigned to the assignee of the present application, entitled xe2x80x9cInterference-Aided Navigation with Temporal Beam Forming in Rotating Vehicles.xe2x80x9d
Others have shown how inputs from multiple antennas may be combined to reduced interference signals. However, for rotating vehicles, it is recommended to utilize roll-independent antennas so that the phase and amplitude adjustments to the antenna signals may be made slowly. Although this approach eliminates the need for rapid control of the interference-cancellation system, the need to utilize a roll-independent antenna limits the effectiveness of cancellation and increases mechanical complexity of the antenna. In general, xe2x80x9croll-independentxe2x80x9d antennas exhibit some residual phase and amplitude modulation with rotation which reduces the effectiveness of the noise cancellation. Also, roll-independent antenna geometries produce circular nulling patterns that that may have undesirable geometries that reduce the strength of desired signals.
Thus, there is a need for an interference-cancellation system that allows the use of simple directional antennas and provides improved jamming immunity. This need is particularly evident with GPS-aided artillery shells and other spinning vehicles operating in high-interference and jammed environments. Further, there is a need to control a roll-dependent phase and amplitude control function allowing antennas with roll-dependent amplitude and phase to be utilized. Even further, there is a need to provide improved jamming and interference resistance, and allow the use of low-cost directional antennas, eliminating problems associated with locating and installing roll-independent antennas. Yet still further, there is a need for an alternative or additional technique to temporal beam forming to improve receiver performance under conditions of high interference or jamming.
The present invention includes embodiments that aid in the acquisition and tracking of a GPS signal. These embodiments may be used alone or combined with other ASVN technologies to provide improved jamming and interference immunity in a wide range of rotating vehicle applications.
In particular, one exemplary embodiment relates to an interference-aided signal acquisition and tracking system. This system includes a vehicle, an interference detector, a noise canceller, and a signal processor. The vehicle has at least two receivers configured to detect external signals and the at least two receivers have an output dependent on attitude of the vehicle. The interference detector measures the output of at least one of the at least two receivers. The noise canceller combines the output of the at least two receivers. The signal processor extracts a desired signal from the output of the noise canceller. The output of the interference detector is used to control the noise canceller as to reject unwanted signals and enhance performance of the signal processor in extracting the desired signal.
Another exemplary embodiment relates to a method of signal acquisition and tracking including receiving external signals at a spinning vehicle, measuring the received external signals, modulating the received external signals to null an interference signal, and extracting a desired signal from the combined external signals.
Still another exemplary embodiment relates to a signal acquisition and tracking system where interference is cancelled for jamming immunity with spinning vehicles operating in interference environments. The system includes a number of signal receivers associated with a spinning vehicle, a rotation tracker that obtains signals from the number of signal receivers and provides a rotation estimate, a global positioning system (GPS) processor that provides satellite geometry information, and an interference cancellation controller that obtains the rotation estimate and the satellite geometry information and modulates to null interference received by the number of signal receivers.